1. Field of the Invention
The invention relates to a spoke for a spoked wheel, in particular a cycle wheel. The invention also relates to a spoked wheel having at least one such spoke, as well as to a rolling apparatus, in particular a cycle, equipped with at least one such wheel.
2. Description of Background and Other Information
The wheel has existed since the mists of time. The first wheels were solid wheels. A first improvement consisted in building compression spoke wheels, which were lighter. The spokes of such wheels were biased in compression and in flexion by a rim. Then, the wheel was ringed by a metal hoop and the compression spokes were pre-stressed in compression. The patent document CH 91759 describes such a mode of construction method.
The invention of the tension wire spoke wheel goes back to around 1866, and is credited to Eugene Meyer. This invention made it possible to build wheels with spokes having a considerably decreased cross section, resulting in a substantial weight reduction. It also became possible to increase the wheel diameter, and therefore to increase its size, as was the case with the steel high-wheels whose pedals were in direct engagement with the driving wheel. The terms “traction” and “tension” are used interchangeably herein to describe a spoke having a positive tension.
Conventionally, a currently available spoke wheel includes a peripheral rim provided to receive a tire, a central hub and connecting spokes between the rim and the hub. The number of spokes is variable depending upon the type of wheels; it generally varies between twelve and forty. As a general rule, the spokes are distributed in two sets, each of which connects the rim to a respective one the ends of the hub. The spokes of each set form an angle with the median plane of the rim, which is commonly referred to as the dish angle.
The spokes structurally connect the rim and the hub, which makes it possible to provide the wheel with good rigidity and good fatigue strength. The external loads to which a wheel is subjected during use can be divided into a radial force directed along the median plane of the rim, a lateral force directed perpendicular to such plane, and a motive force or, conversely, a braking force tangent to the wheel circle, which corresponds to the transmission of torque between the hub and the rim.
Constructions of wheels other than tension spoke wheels currently exist. For example, solid wheels or so-called compression spoke wheels are known, which are made out of composite material and are used mainly for their aerodynamic properties. The patent documents WO 2004/033231 and FR 2701899 describe such wheels. There are also molded wheels made out of light alloy (aluminum, magnesium or titanium). Such wheels are known, for example, from patent documents EP 1016552 and WO 2004/108515.
However, among these various wheel modes of construction, the tension spoke wheel still offers the best compromise between lightness and strength, provided that it is well built and properly adjusted.
It is commonly believed that the more tensioned the spokes, the more rigid the wheel. However, this belief is erroneous because excessive tension in the spokes in fact makes the wheel more flexible and also weakens it. Indeed, the risk of causing the rim to buckle under the compressive stress produced by the spokes increases substantially if the spokes are overly tensioned, i.e., overly tightened. Another problem related to excessive spoke tension is the variation in the wheel diameter before and after tensioning. Contrary to the common preconceived notion that the wheel spokes must be tensioned to the maximum, one of the problems associated with tension spoke wheels therefore resides in the application of correct, not excessive, tension.
In general, it is believed that spoke tension must be sufficient, so that none of the spokes becomes loose during normal use of the wheel. Indeed, a spoke that becomes temporarily loose becomes non-existent in relation to the rim and the other spokes; and the wheel consequently loses rigidity locally.
The rim, the hub, and the spokes must be considered as a structure in which the forces are balanced. The tension of each spoke is taken up by the hub, the rim, and the other spokes. A force applied to the hub or the rim is reflected on all of the spokes. For a rear wheel, it is also necessary to take into account the tension level that is different in the spokes located on the side of the freewheel and the spokes located on the side opposite the freewheel, due to the difference in the dish angle between the two sets. Finally, depending upon the orientation of the spokes in the set, and depending upon whether the spoking pattern is radial or crossed, in particular, the spoke tension can be different. When a spoked wheel is built, the spokes are sufficiently tensioned so as not to become loose during normal use.
Thus, it is generally believed that a wheel must be capable of withstanding the following loads without any spoke loosening:                at least 1500 Newtons of radial force for a rear wheel, 1200 Newtons for a front wheel,        at least 200 Newtons of lateral force,        at least 150 Newton-meters (Nm) of drive torque for the rear wheel,        at least 300 Nm of braking torque for a front wheel with a brake system on the hub, and 150 Nm for a rear wheel of the same type.        
These values are given for information only and are not limiting. Indeed, they depend on the activity involved and also on the size of the cyclist.
Another reason that a spoke must be constantly tensioned is that a spoke has a very small cross section compared to its length. If the tension becomes negative, i.e., if the spoke stress turns into compression, the spoke buckles immediately.
A tension spoke wheel yields good results, but nevertheless has several disadvantages.
First, the spoke tension produces compressive stresses in the rim body. It is estimated that for a road bike wheel having 36 spokes, with each spoke being tensioned to 1000 Newtons, the compressive force resulting in the area of the rim body is 5730 Newtons, which results in a compressive force of 88 MegaPascals (MPa), which, for a material commonly used for a rim (aluminum 6106, for example), already represents 40% of the material inherent strength potential (220 MPa). In other words, this resultant compression already weakens the rim considerably.
In addition, for a rim 622 millimeters (mm) in diameter, this compressive force leads to a reduction in the rim perimeter of up to 2.5 mm, which, naturally, can have an impact on the connection between the rim and the tire, and can lead to ill-timed tire roll offs and accidents.
Second, the spoke tension is exerted on the rim locally. Each spoke, via its tension, produces a local shearing force in the area of its attachment zone, as well a variable bending moment between each spoke hole. The bending moment leads to a polygonal deformation of the rim, commonly referred to as a “jump”, with a local lateral run-out or deformation in the area of each spoke attachment zone.
The patent documents EP 1316442 and FR 1019285, providing for paired spoking patterns, illustrate these two phenomena for which they attempt to find a solution. It is noted in passing that attaching the spokes by pairs in the area of the rim, as described in the document EP 1316442, does attenuate the effect of lateral run-out, but accentuates the polygonal effect.
To solve this problem, EP 1316442 proposes to start with a rim that is pre-deformed in an opposite configuration (see FIG. 5 of this patent), which is difficult to implement.
Lastly, it has been observed that the service life of a wheel, i.e., of each of its components, is substantially inversely proportional to the tension of the spokes. During wheel rotation, each spoke is subject to a loading and unloading cycle, and each spoke locally subjects the rim to such a loading and unloading cycle. These repeated cycles result in damage to the spoke or the rim, and this all the more quickly as the tension in the spoke is high. Consequently, the currently available tension spoke wheels do not have an optimal service life. In view of this state of the art, there exists a need for a wheel construction that reconciles rigidity, strength, and optimal service life.
There also exists a need for a wheel whose geometrical characteristics (lateral run-out or deformation, jump, perimeter) remain as stable as possible.